Corrosion and deterioration of concrete pipes, manholes, wet wells, chambers, tunnels, diversion boxes, pump stations, drop structure reservoirs and treatment basins due to sulfuric acid attack is a major concern associated with wastewater conveyance and treatment facilities. Traditional cementitious materials such as Portland cement are inexpensive, but do not offer longevity under wastewater conveyance and treatment conditions. Concrete pipes are chemically attacked when subjected to acids with pH values of 6.5 or lower for extended periods of time. The pH in sewer lines can reach values of 2 or 3, and in some extreme cases 0.5. The highly acidic environment in sewer pipe lines and wastewater treatment facilities significantly reduces the life of these buried structures, causing significant financial losses.
Efforts have been made to address issues with concrete and brick surfaces in wastewater collection and treatment systems such as susceptibility to corrosion, cracking, and lack of long-term durability in harsh environments. For example, additives have been added to Portland cement in an effort to enhance the corrosion resistance of the Portland cement. Attempted additives include silica fume, fly ash, and blast furnace slag. These additives react with Ca(OH)2 present in cement paste to produce C—S—H, which enhances the resistance of the hardened cement paste in environments with pH values above 4.5. Another example of an attempted method of protecting concrete surfaces is the addition of a thin layer of chemically resistant material (e.g., polyurethane, polyurea, epoxy, mortar epoxy, high alumina cement, or asphalt) on the inner surface of concrete pipes or other concrete surfaces. Difficulties with the addition of these thin layers include issues with ensuring adequate bonds between a spray-on coating and the host concrete surface, formation of pinholes that allow sulfuric acid and/or bacteria to penetrate the coating and destroy the bond between the coating and the host concrete surface, ensuring proper coverage at joints of concrete pipes, and construction related damage to the coating during installation. Also, both of these efforts significantly increased costs of construction and operation.
Geopolymers are inorganic alumino-silicate amorphous polymers formed by chemical reactions under highly alkaline conditions between an active pozzolanic material, such as fly ash or metakaolin, and an activator solution (e.g., a mixture of sodium hydroxide and an alkaline silicate such as sodium silicate or potassium silicate). Polymeric chains form when a pozzolanic material comes in contact with an alkaline activator solution. The geopolymer net consists of SiO4 and AlO4 tetrahedra linked together by shared oxygen atoms. Inside the cavities of the geopolymer net, positive ions (e.g., Na+, K+, Li+, Ca2+, Ba2+, NH4+, and H30+) should be present to balance the negative charge of Al3+ so that the aluminum atom can be linked to four oxygen atoms. The following is the empirical formula for geopolymer polysialates:Mn(—(SiO2)z—AlO2)n.wH2O,where M is any of the above-mentioned cations, n is the degree of polymerization, z is 1, 2, or 3 indicating the type of geopolymer formed, and w is the number of associated water molecules. For z=1, the net will be of the polysialate type. For z=2, the net will be a poly(sialate-siloxo) type. For z=3, the net will be a poly(sialate-disiloxo) type.
Geopolymers exhibit excellent compressive resistance (up to 120 MPa) and rapid strength gain, with 95% of their ultimate strength achieved in as little as three days under proper curing conditions. Geopolymers also exhibit low vulnerability to chemical attacks, and are practically inert to attack by sulfate salts because they are not based on calcium silicate. Because they are composed of an alkaline silicate net, geopolymers are also inert to alkali-aggregate reaction, which is a common concern with Portland cement.